Prediction of the frequency and duration of ambulatory myocardial ischemia in patients with stable coronary artery disease by determination of the ischemic threshold from exercise testing: Importance of the exercise protocol

The relation between ambulatory myocardial ischemia and the results of exercise testing in patients with ischemic heart disease remains undefined, because of the dissimilar results of previous reports. To further investigate this issue and, in particular, to ascertain the importance of the exercise protocol in determining that relation, 70 patients with stable coronary artery disease underwent 48 h ambulatory electrocardiographic (ECG) monitoring and treadmill exercise tests after withdrawal of medications.Patients exercised using two different protocols with slow (National Institutes of Health [NIH] combined protocol) and brisk (Bruce protocol) work load increments. Exercise duration was longer with the NIH combined protocol (14.1 ± 5 versus 6.8 ± 2 min; p < 0.0001), but the maximal work load and peak heart rate achieved were greater with the Bruce protocol (9.8 ± 2 versus 6.5 ± 2 METs, and 142 ± 19 versus 133 ± 22 beats/min, respectively; p < 0.0001). A close inverse correlation between exercise testing and the results of ambulatory ECG monitoring was observed using the NIH combined protocol; the strongest correlation was observed between time of exercise at 1 mm of ST segment depression and number of ischemic episodes (r = −0.86; p < 0.0001). With the Bruce protocol a significantly weaker inverse correlation was found (r = −0.35). The mean heart rate at the onset of ST segment depression was similar during monitoring and during exercise testing with the NIH combined protocol (97.2 ± 13 versus 101.0 ± 17 beats/min, respectively) but it was significantly higher (110.4 ± 13) when using the Bruce protocol (p > 0.001).These findings indicate that a relation does exist between ambulatory myocardial ischemia and the results of the exercise test but this relation is critically determined by the exercise protocol and is better observed with protocols that produce slow work load increments. These observations raise questions about the independent value and usefulness of ambulatory ECG monitoring in patients with stable ischemic heart disease